the exemplary case of .sup.14 N, it has been
Turning first to proposed to utilize the phenomenon of the sharp nuclear resonance provided by excitation energy of 9.172 MeV in .sup.14 N as a unique and clear signature for detecting the presence of nitrogen as in explosives that may, for example, be hidden in luggage. The gamma-ray transition rate from the ground state of .sup.14 N to this particular excited state is quite large, such that gamma rays of 9.172 MeV are very strongly absorbed by the ordinary nitrogen nuclei and thus provide a clear indication of the presence of nitrogen. Through the inverse reaction for generating such gamma rays as a probing source of detecting nitrogen--i.e., impacting an appropriate energy proton beam upon carbon 13 .sup.13 C), the reaction .sup.13 C (p,.gamma.).sup.14 N occurs, resulting in the generation of gamma rays with such 9.172 MeV energy at an angle of 80.5.degree. to the proton beam direction, useful for subsequent resonance absorption in nitrogen-containing media probed by the gamma rays. The use of such resonance gamma-ray absorption to detect explosives in luggage or for other nitrogen-detecting purposes, has accordingly been suggested.
This is particularly interesting because of the sharpness of the resonance, the significant absorption probability (large integrated cross section) and the specificity to nitrogen high detection sensitivity with concomitant important insensitivity to troublesome background radiation and materials. In addition, the high penetrating properties in ordinary materials renders the probing by 9.172 MeV gamma rays substantially impervious to attempts to shield the explosive to avoid detection.
Considering the application of the invention to the inspection of luggage or the like for explosives, security of airports against clandestine bombs is of paramount importance in air travel. A substantial effort has been underway in this country and abroad to develop methods that will find hidden explosives carried on board airlines in luggage. Nuclear methods appear to be the only ones capable of testing bags for small amounts of explosive materials, rapidly, reliably and non-destructively.
One system, Thermal Neutron Analysis (TNA), is now commercially available. But TNA has many drawbacks. It is relatively slow; its sensitivity is limited; explosives may be camouflaged; and it makes the luggage radioactive. One of the alternative schemes being developed by scientists in Israel, overcomes most of the drawbacks of TNA. The Israeli method, earlier mentioned as Resonance Absorption Analysis (RAA), makes use of the before-described special resonance in the nucleus of nitrogen, a ubiquitous component of all high-performance explosives. The resonance is excited by high energy gamma rays that are passed through the examined luggage, the gamma rays themselves being producable, as previously stated, by a low energy proton beam. The system generates no radioactivity, is impossible to camouflage, results in fewer false alarms, and is, in principle, much more sensitive to small and thin explosives.
While a strong indicator of the presence of explosives, such use of nitrogen RAA is not always conclusive identification of an explosive--there being also other nitrogen-containing materials transportable in luggage and otherwise. Explosives, however, can be uniquely separated from non-explosives by measuring the oxygen concentration, again by RAA techniques, at the same time as the nitrogen RAA, and correlating the nitrogen density of a probed material with the oxygen density.
This feature of the present invention, including novel oxygen RAA techniques and resonant absorption detection, later described, enables the use of the same proton beam for the RAA of oxygen as is used for the RAA of nitrogen. In this way, a single accelerator--the largest single cost of an RAA scheme--is enabled simultaneously to measure both the oxygen and nitrogen distributions in a piece of luggage.
Since all explosive materials have high densities, typically one and half times that of water, they have relatively high nitrogen and oxygen concentrations and relatively low carbon and hydrogen concentrations. No single characteristic is unique though many common non-explosive materials have similar densities, or nitrogen concentrations. But a much smaller sub-set of materials has the high nitrogen density of explosives, and almost no common materials have both the high nitrogen and oxygen densities that characterize all explosives. While the reliable measurement of the distribution of nitrogen densities inside a bag provides an assured deterrent against explosives with few false alarms, the measurement of distributions of both the nitrogen and the oxygen inside a bag, provides that security with almost no false alarms.
Returning to the consideration of the large cost of the accelerator or generator of the proton energy, current accelerator beam requirements are at or near the limits of present day technology. If the beam requirements can be reduced by a factor of five, however, it will permit the use of off-the-shelf accelerators and thus become of practical promise.
The present invention as applied more particularly to the nitrogen RAA, enables improvement by a factor of at least ten; and in principle, it will allow the beam current to be reduced by as much as a factor of 100 without diminishing the signal strength; such being achieved by successively increasing the energy of the incident charged particle beam in the target medium at the same rate that the charged particles lose energy by collisions with corresponding successive portions of the target medium; as resonance-produced gamma rays are generated at such successive portions. This enables the use of practical proton beam sources and renders RAA practically and commercially feasible.
In the case of nitrogen, as previously stated, gamma rays of precisely 9.172 MeV will be resonantly absorbed by the nitrogen. The only practical way of creating these gamma rays is to make use of this resonant reaction itself. This is done, as earlier described, by bombarding the carbon target isotope with protons--such isotope of carbon with 7 neutrons being previously abbreviated as .sup.13 C. The protons in the impinging beam must have precisely 1,747,600 electron volts of energy (within the present uncertainty of measurement) to create the resonant gamma ray of 9.172 Mev. If the protons have 150 electron volts too much or too little energy, the resonance will be missed and the reaction will not take place effectively.
Protons, on passing through the carbon target, will lose energy by collisions with the electrons and nuclei of the carbon atoms. Protons of 1.7 Mev lose approximately 300 electron volts on traversing about 100 Angstroms of a carbon foil. The useful thickness of the carbon target is, therefore, only about 50 atomic layers, which results in an energy loss of 150 electron volts. That is a very thin target. (Nuclear physicists use the units of micrograms per square centimeter for describing target thicknesses; 500 Angstroms of carbon foil being about 1 .mu.g/cm.sup.2). In practice, targets 70 .mu.g/cm.sub.2 thick may be evaporated on thick copper blocks since these are easier to make and cool. The energy of the proton beam incident on the target has an energy somewhat above 9.172 Mev and loses energy in the layers of the thick carbon target until it has the right resonant energy for the reaction to take place.
As before stated, by successively restoring the energy loss suffered by collisions with successive foils of a target, for example, the invention enables presently available proton accelerators to be used.
It is to adapting the above-described technique for use with such practical proton sources, that the present invention is, in one of its important aspects, particularly directed. Specifically, by replacing the energy lost by the protons directed through the .sup.13 C-containing target medium portion, such as a first thin carbon layer, there is restored the original capability of the proton beam to create a second .sup.14 N resonance-produced 9.172 MeV gamma ray generation in a next successive juxtaposed .sup.13 C-containing target portion, such as a next thin carbon layer; and so on, in seriation--each time adding appropriate voltage at each successive target portion, thereby enhancing the yield of the required gamma rays, all with the original, relatively low energy proton source. Thus the proton energy available at each successive portion of the target is rendered substantially the same as the original proton energy from the accelerator source impinged upon the first portion of the target, restoring the probability of gamma ray production at each successive target portion (or carbon foil in the above example). This successive voltage injection and energy compensation technique is also applicable, as later explained, to gaseous targets such a continuous gaseous target or an array of separate gaseous .sup.13 C-containing cells, as well.
As will later be more fully explained, the principle hereinvolved is applicable, also, to other elements than nitrogen and their corresponding targets, including as an illustration chlorine, which is also a constituent of some explosives; and combinations of such elements may also be detected in accordance with the invention.
It is accordingly an object of the invention to provide a new and improved method of and apparatus for resonance absorption measurements of both nitrogen and oxygen in objects, including in luggage and the like, substantially uniquely to detect explosives and the like through correlation of detected nitrogen density of the media with oxygen density.
A further object of the invention is to provide a novel method of and apparatus for oxygen RAA and for resonance absorption detection of resonance-produced gamma rays.
Still another object of the invention is to provide a new and improved method of and apparatus for increasing the gamma ray yield obtained in nuclear resonant reactions caused by charged particle beams, including for such purposes as the detection of constituent elements of explosives in luggage and the like, and for similar or related objectives, as well.
Another object is to provide a new and improved method of and apparatus for enhancing .sup.14 N-resonance-produced 9.17 MeV gamma ray generation by proton impingement on .sup.13 C-containing media.
Still an additional object is to provide for such enhancement of gamma ray yield from such nuclear resonant reactions in chlorine and other appropriate elements as well.
An additional object is to provide new and improved target structures particularly suited to the practice of such resonance absorption enhancement or yield-increasing methods, and useful also for other applications.
Other and further objects will be described hereinafter and are more particularly pointed out in the appended claims.